Neutron well logging is a well developed art and neutron well logging systems are described, for example, in U.S. Pat. Nos. 3,146,351 (Hoyer et al); 3,379,882 (Youmans); 3,379,884 (Youmans); 3,461,291 (Goodman); 3,714,468 (Hopkinson); and 3,924,138 (Hopkinson). Neutron tubes used in borehole logging probes require target voltages of on the order of one hundred thousand volts at average currents of about one milliampere. The tube and associated power supply assemblies must be of small diameter, typically two inches or less. Most applications require a burst of neutrons lasting about ten microseconds with repetition rates of from hundreds to thousands of pulses per second. The target voltage needs to be high only during the pulse, at which time the peak current is several hundred milliamperes.
Conventional power supplies for neutron tubes may be divided into two different types: D.C. and "full pulse". An example of the former type is disclosed in the Hopkinson patents referred to above. The latter type is disclosed, for example, in Bivens et al., Proceedings of the Fourth Conference on the Scientific and Industrial Applications of Small Accelerators (October 1976), pp. 441-446, "Pulsed Neutron Uranium Borehole Logging with Prompt Fission Neutrons", IEEE Publication No. 76 CH 1175-9 NPS.
DC supplies suffer several disadvantages. For example, they are large in size - with typical supplies being about three or four feet in length. Further, there are insulation limitations, with D.C. supplies requiring more insulation than pulsed voltage supplies. Because of this factor, the practical DC limit for a two-inch diameter assembly appears to be about 100 kV, whereas a deuterium-tritium neutron tube operates most efficiently at about 120 kV. In addition, neutron tubes are more prone to internal breakdown for DC voltages than for pulsed voltages. For example, the Zetatron manufactured by Sandia National Laboratories is limited to 80 kV DC but can be operated above 130 kV in the pulsed mode. On the other hand, the major advantage of a DC supply is that it will permit the associated neutron tube to operate at a higher repetition rate, since a DC supply is more efficient than a pulsed supply.
A full-pulse supply brings the target voltage from zero to full voltage and back to zero again in a few tens of microseconds, usually with the use of a single pulse transformer. This reduces the high voltage part of the supply to a length of less than six inches, thus allowing the tube and transformer to be "potted" together in a single convenient assembly or unit. Further, as mentioned above, a full-pulse supply does not stress the insulation and tube as much as a corresponding DC voltage. However, there are disadvantages with the full-pulse method. First, bringing the target from zero to full voltage during each pulse requires that the transformer dissipate more heat energy than with a DC supply, and this heat is concentrated in a smaller volume. The resulting temperature rise limits the repetition rate of the tube-transformer assembly to about 100 HZ and shortens the life of the assembly. Further, placing the 120-kV pulse transformer associated with the neutron tube into a two-inch diameter space results in a high insulation stress at various parts of the secondary winding and thereby limits the operating life of the transformer. In addition, after the main negative pulse is generated, the transformer voltage usually overshoots zero and applies a reverse (positive) voltage to the tube. This reverse voltage can reach tens of kilovolts and can cause breakdowns which shorten the life of the assembly.